1. Field of the Invention
The present invention relates to electronic equipment, and more particularly to an electronic equipment which is selectively powered by an internal battery power supply and an external power supply connected through an external connecting terminal, in accordance with the setting of a selection switch.
2. Description of the Prior Art
In prior art electronic equipment which adopts a two-way power supply system, such as an electronic desk top calculator, either a battery power supply or an external power supply is selected by a power switch to operate an arithmetic unit and other circuits including a thermal printer. In such electronic equipment, when the power supply is switched from an old battery to the external power supply, a voltage difference therebetween may cause an adverse effect on the operation. For example, in a modern desk top calculator having a thermal printer, a power supply voltage for driving the thermal printer is measured by an arithmetic unit and a heating pulse for driving the thermal head having a duration corresponding to the measured power supply voltage is generated to keep print density at a constant level. In such a drive system, if the power supply voltage changes abruptly after the power supply voltage has been measured and the heating pulse duration for the thermal head has been determined, the operation of the thermal head is adversely affected.
For example, when the measured battery voltage is low because of long usage of the battery, the thermal head is driven by a long duration heating pulse in order to attain the constant level of the print density. If the power supply is switched by a power switch to an AC adaptor power supply having a higher voltage after the measurement of the battery voltage, the thermal head is driven by the long duration heat pulse with the higher voltage and, therefore, the thermal head may be broken.
When the power supply is switched by the power switch from the AC adaptor power supply to a new higher voltage battery power supply after the measurement of the voltage of the AC adaptor power supply, the thermal head may also be broken.